1. Field of the Invention
The present invention relates to circuits for sensing and indicating the temperature of an integrated circuit, and in particular, to such temperature indicator circuits having temperature hysteresis.
2. Description of the Related Art
Uses of temperature sensing circuitry within integrated circuits have been on the rise and continue to increase. Reasons for this include desires to monitor the temperature of chips which, due to operations at significantly increased clock speeds, often operate at temperatures which, depending upon the operating environment, can approach or even exceed maximum temperature limits or cause serious degradation in the performance characteristics of such chips.
Additionally, the users of some chips wish to know the temperature of the chip so as to plan for and implement power reduction techniques, such as reducing clock speed, prior to reaching some predetermined temperature limit. By knowing the temperature of the chip prior to reaching the predetermined limit, plans can be made for implementation of power reduction techniques, such as reducing or changing the level or type of data processing being performed prior to the planned reduction in clock speed. This allows the overall temperature of the chip to be reduced by slowing down or otherwise modifying its operating characteristics without requiring that the chip instead be temporarily disabled.
Referring to FIG. 1, a conventional approach to generating a signal VOUT which corresponds to the temperature being monitored is to use a voltage comparator 10 with hysteresis (in terms of when the output VOUT changes state due to a change in the relative values of the input signals VREF and VX). The reference voltage VREF is typically the output of a digital-to-analog converter (DAC) and is generally quite stable over temperature variations. The temperature signal VX is a voltage generated by a temperature sensor circuit and varies substantially linearly over variations in temperature. Such a temperature signal VX has either a positive or negative temperature coefficient. In this example, temperature voltage VX is presumed to have a negative temperature coefficient.
In such an implementation, the hysteresis of the comparator 10 is critical due to the difficulty in maintaining the consistency of the window temperature error, typically in the range of five degrees Celsius. In order to achieve and maintain such a small and consistent hysteresis window, it is often necessary to process numerous iterations of the integrated circuit in which such a circuit is to be used so as to ensure such consistency regardless of variations in fabrication properties, power, etc.
Conventional techniques for generating the temperature-dependent voltage VX include: monitoring the voltage drop Vbe across a forward biased base-emitter junction of a bipolar junction transistor (BJT); monitoring the reverse breakdown voltage of the base-emitter diode of a BJT; monitoring the voltage from the collector to the emitter of a current saturated BJT; or monitoring the ratio of diffused resistors within the integrated circuit. While each of these techniques are useful in that they rely upon predictable device parameters, a significant disadvantage to such techniques is that the dynamic range of the change in voltage over a useful temperature range is quite small. Accordingly, when comparing such a voltage with such a small dynamic range against a reference voltage generated by an N-bit DAC, the voltage of such DAC which corresponds to its least significant bit (LSB) is very small. This makes the DAC difficult to design and maintain with respect to consistency of its performance characteristics.
Accordingly, it would be desirable to have a circuit for sensing and indicating substrate temperatures which overcomes such problems.